/**
 * Created with IntelliJ IDEA.
 * Description:
 * Date: 2025-04-07
 * Time: 10:59
 */
public class Main {
    public int tribonacci(int n) {
        if(n==0){
            return 0;
        }
        if(n==1||n==2){
            return 1;
        }

        int[] dp = new int[n+1];
        dp[0] = 0;
        dp[1] = dp[2] = 1;

        for(int i = 3;i <= n;i++){
            dp[i] = dp[i-1]+dp[i-2]+dp[i-3];
        }
        return dp[n];
    }
    public int tribonacci1(int n) {
        //空间优化（背包问题）
        if(n==0){
            return 0;
        }
        if(n==1||n==2){
            return 1;
        }

        int a = 0,b = 1,c = 1,d = 0;

        for(int i = 3;i <= n;i++){
            d = a + b + c;
            a = b; b = c; c = d;
        }
        return d;
    }
}
